The increasing market demand for wireless connectivity coupled with innovations in integrated circuit technology has resulted in increased research and development of low cost, low power, and compact monolithic integration of radio transmitters, receivers, and transceivers capable of operating at GHz carrier frequencies. For example, advances in high-speed silicon-germanium technology, together with the availability for unlicensed use of 3 GHz-7 GHz of bandwidth in the 60 GHz ISM (industrial-scientific-medical) band, have served as a catalyst for potential widespread implementation of low cost 60 GHz wireless data transmission systems using integrated 60 GHz radio transceivers.
Indeed, the large unlicensed bandwidth available in the 60 GHz ISM band enables wireless systems to be implemented using digital modulation techniques such as QAM (quadrature amplitude modulation), ASK (amplitude-shift key), FSK (frequency-shift key) or PSK (phase-shift key) modulations to achieve gigabit-rate modulations that support high bandwidth applications such as wireless Gigabit Ethernet and HDTV (high definition television) streaming. In comparison to conventional analog modulation techniques, digital modulation schemes such as QAM, ASK, FSK and PSK are more power efficient and are more robust to noise and multipath effects, etc. Moreover, digital modulation provides more information capacity, higher data security, and better quality communication, especially for low power, high data rate applications.
The quadrature modulator is a fundamental component that is commonly used for digital microwave radio communications systems. FIG. 1 is a block diagram illustrating a quadrature modulator (10) having a conventional framework. The quadrature modulator (10) comprises a first mixer (11), a second mixer (12), a phase shifter (13) and summing circuit (14). The inputs to the quadrature modulator (10) include modulating signals I (in-phase signal) and Q (quadrature-phase signal), and an LO (local oscillator) signal. The I and Q modulating signals may be analog signals I(t) and Q(t) generated by a baseband processor which implements complex DSP processing functions for purposes of processing and encoding a data signal (digital data) to generate independent I and Q serialized digital data streams to represent the input data. The baseband processor can implement DAC (digital to analog converters) to covert the I and Q digital streams to analog signals I(t) and Q(t), which are input to respective IQ mixers (11) and (12).
The LO port of the first mixer (11) receives the LO signal as an in-phase LO signal, LO1(0°), while the phase shifter (13) phase shifts the LO signal by 90° to produce a quadrature-phase LO signal, LOQ (90°), which is input to the LO port of the second mixer (12). The first mixer (11) mixes the I-channel input signal, I(t), with the in-phase LO signal, LO1, and the second mixer (12) mixes the Q-channel input signal, Q(t), with the quadrature-phase LO signal, LOQ. The outputs of the mixers (11) and (12) are combined by the summing circuit (14) to generate a QAM modulated output:Vout(t)=[I(t)*cos(2πfLOt)]−[Q(t)*sin(2πfLOt)]  (1)where I(t)=A(t)cos φ(t) and Q(t)=A(t)sin φ(t).
The QAM output signal of the modulator (10) can also be represented as:Vout(t)=A(t)cos [(wLOt)−φ(t)]  (2),where the signal amplitude isA(t)=√{square root over (I2(t)+Q2(t))}{square root over (I2(t)+Q2(t))}  (3)and where the phase modulation (shift) isφ(t)=tan−1[Q(t)/I(t)]  (4)
In this regard, a conventional quadrature modulator such as depicted in FIG. 1 can support a modulation scheme in which one or more parameters of the carrier (LO) frequency (amplitude, phase and/or frequency) can be modulated to represent information. The quadrature modulator (10) of FIG. 1 can readily support various QAM modulation modes and PSK modulation modes, such as BPSK (binary phase-shift key) (which is essentially 2-QAM) and QPSK (quadrature phase-shift key) modulation (which is essentially 4-QAM). In theory, any modulation format can be implemented using quadrature (IQ) modulation techniques, including ASK and FSK, by converting an input stream of data bits to appropriate waveform values on the baseband I and Q inputs to realize the desired modulation, as is understood by those of ordinary skill in the art. When using quadrature modulation schemes, however, it is a non-trivial, challenging and resource-consuming task to create the required baseband waveforms for a desired modulation (e.g., FSK) at the very high speeds that are required to achieve gigabit rate modulation.